[linalg.conj.conjugatedaccessor] Fix incorrect return type of nested_accessor() (#8105)

Author: Eisenwave

source/numerics.tex

@@ -12802,7 +12802,7 @@
     constexpr typename offset_policy::data_handle_type
       offset(data_handle_type p, size_t i) const;
 
-    constexpr const Accessor& nested_accessor() const noexcept { return @\exposid{nested-accessor_}@; }
+    constexpr const NestedAccessor& nested_accessor() const noexcept { return @\exposid{nested-accessor_}@; }
 
   private:
     NestedAccessor @\exposid{nested-accessor_}@{};                           // \expos